1. Field of the Invention
The present invention relates to an image writing apparatus that uses a light-emitting device array.
2. Description of the Related Art
In an image writing apparatus that writes a latent image on a photosensitive member by irradiating light, a laser diode (LD) scanning system and a system (light-emitting-device array system) using a light-emitting-device array are used. The light-emitting-device array system uses light-emitting diodes (LED) arranged in an array. In the light-emitting-device array system, when an image forming apparatus outputs a binary image, a one-dot character is printed in an elliptical shape, which is long in a horizontal direction, depending on process conditions. In a one-dot lattice image (an image with dot intervals of five millimeters), the elliptical shape appears clearly. As a result, vertical lines are printed thicker than horizontal lines to cause a problem in an aspect ratio.
To solve the problem, there is a system for controlling the aspect ratio using balance correction data for LEDs. As a system using multiple value data, gradation data for each LED, data for correcting output fluctuation by a unit of block including plural LEDs, and data for correcting output fluctuation in an average value of blocks are added. Fluctuation in LEDs is controlled based on added data.
As a system using two values, there is control for faithfully reproducing gradation by adding data of a binary image and correction data of respective LEDs. Since print power of dots (print drive current control) is adjusted, a line image is improved. However, the aspect ratio is still not improved.
Some LED light-emitting device arrays do not use the system for adding correction data and print image data.
In a technology described in Japanese Patent Application No. 2003-412065, an LED writing control circuit controls LEDs by lighting the LEDs for several times. The LEDs are lit twice, respectively, when an image is transferred once in a light-emitting-device array unit that transfers odd-number image data after transferring even-number image data.
As another method, there is a system for transferring image data several times in one line to control a lighting signal. In this system, one-line data is transferred several times in one line to control the lighting signal and perform printing. Consequently, it is possible to faithfully print a line image formed with oblique lines. Although an aspect ratio of a printed image is not improved, since thickness of horizontal lines is increased, a ratio of vertical lines and horizontal lines is improved.
As another technology, there is a system for transferring image data several times in one line and recognizing a one-dot isolated point in a main scanning direction to control data processing. In this system, data transfer is performed several times (twice), a pattern is recognized in the main scanning direction, and when the one-dot isolated point is found, data is processed to be changed from “1” to “0” to reduce a vertical line width and improve vertical and horizontal line widths.
When data transfer, data processing, and lighting time are controlled to reduce a vertical line width and clarify a horizontal line width, more accurately, a ratio of vertical and horizontal line widths, it is possible to obtain an image writing apparatus with improved performance. For example, when an image density is 600 dots per inch (dpi), a dot diameter is 42.33 micrometers (mm). The dot diameter slightly increases depending on process conditions and fixing properties. However, the dot diameter has to be set as close as possible to the dot diameter in the case of 600 dpi.